Sludges derived from industrial clarification plants, communal sewage plants and various chemical engineering processes have to be dehydrated before being disposed of or finally used in order to be subsequently exploitable. Such exploitation, for instance, involves combustion, composting or even agricultural applications. In this connection, increasing transportation costs involved in sludge disposal have in the past called for the dehydration of the occurring sludges to high final solids contents in order to make them reusable for subsequent exploitations. To this end, polymer coagulation or flocculation aids have been added to the most diverse sludges in order to form flake-containing sludges that will subsequently be more readily dehydratable.
For continuous dehydration, the sludge together with the flocculation aid is applied onto a screw extruder from a feed side and continuously dehydrated to the desired degree of drying or dehydration of the sludge by conveying the latter through the extruder while increasing the dehydration pressure. Since, with such a screw extruder, the solids concentration of the sludge is low on the side of the feed opening into the extruder and continuously increases towards the discharge side, a large amount of water is to be immediately discharged in the region of the feed opening, which filtrate water, unlike the water present in screw extruder regions located farther away from the feed opening, is usually only little contaminated, carrying along only extremely small amounts of solid particles. Despite this known fact that most of the filtrate water occurs in the region of the feed opening, the filtrate water in the screw extruders of the prior art is collected in one and the same filtrate trough, or one filtrate trough, to be subsequently reintroduced into the process and, in particular, into the crude sludge.
Thus, screw extruders are, for instance, known, in which the screw shaft is mounted so as to be inclined relative to the horizontal line, and in which, in particular, the feed side is located deeper than the discharge side of the screw shaft so as to enforce, by the inclined disposition, additional conveyance of the filtrate water outflow in the direction to the feed side.
From EP-B 1 148 990, a screw extruder is, for instance, furthermore known, in which a device for preventing the obstruction of the filter zone on the feed side of the extruder is configured such that the filter tube and/or the filter sieve is/are driven to rotation, particularly on the feed side, in order to prevent an obstruction of the filter sieve, on the one hand, and to ensure the safe evacuation of excessive amounts of filtrate water in that region, on the other hand.
In addition, the most diverse screw extruders have already been provided, in which the mesh sizes of the filter sieves have been variably adaptable in order to be adapted to the respective degree of dehydration of the filter material and be able to safely cope with the discharge of the occurring amounts of filtrate in the feed region of the filter press, wherein all known screw extruders have in common that, although as complete and efficient a dehydration of the filter sludge is sought, the occurring filtrate, irrespectively of its composition, is in any event immediately returned to the process, or discarded, without trying to improve the overall energy balance of the screw extruder or feed the filtrate to immediate further use.
The present invention now aims to provide a screw extruder of the initially defined kind, by which it will be possible to not only safely and reliably dehydrate sludge or crude liquid to be dehydrated, but at the same time also separate the occurring filtrate in such a manner that, on the one hand, the filtrate can be immediately returned into the process and, on the other hand, those amounts of filtrate which may be used for other purposes can be directly discharged from the process, and further used, without additional purification or treatment.